(1) Field of the Invention
This invention relates to an impact-resistant methacrylic resin which has excellent transparency and weatherability as well as good surface gloss. More specifically, it relates to an impact-resistant methacrylic resin having a good surface gloss, which comprises a methacrylic resin and, incorporated therein, a graft copolymer obtained by graft-polymerizing a monomer composed mainly of methyl methacrylate to an acrylic elastomer composed mainly of an acrylic acid ester, which has been crosslinked with a substance having a special structure.
(2) Description of the Prior Art
Among plastic materials, a methacrylic resin has excellent transparency and optical characteristics and also an excellent surface gloss, weatherability, dye or pigment colorability, and moldability. Accordingly, this resin is widely used for illumination articles, signboards, window materials, optical lenses, vehicle parts, and display devices. However, this methacrylic resin is defective in that the impact resistance is poor, and thus, improvement of the impact resistance is strongly desired in the respective fields in which the resin is used. Various methods for imparting a good impact resistance to a methacrylic resin have been proposed. However, an impact-resistant methacrylic resin which retains excellent inherent characteristics such as high transparency, good appearance, high weatherability, and good moldability has not been developed.
According to most of these known methods, an elastomer which is rubbery at normal temperature is dispersed in a methacrylic resin. As the rubbery elastomer, there have been used an unsaturated rubber elastomer composed mainly of butadiene, and a saturated rubbery elastomer such as an acrylic acid ester copolymer composed mainly of butyl acrylate or 2-ethyl-hexyl acrylate, or an ethylene/vinyl acetate copolymer.
The incorporation of an unsaturated rubber elastomer in a methacrylic resin results in the manifestation of a high impact resistance, but the weatherability is degraded by the unsaturated bonds present in the main chain of the polymer. A good weatherability is attained by the incorporation of a saturated rubbery elastomer, but since the modulus of elasticity and elastic recovery of the saturated rubbery elastomer per se are low and the elastomer has a poor graft-polymerizability with a hard resin component, the impact resistance, transparency, and surface gloss are poor. Further, the surface appearance is not satisfactory because of flow marks.
Important factors in preparing a two-component type impact-resistant resin comprising a discontinuous phase of particles of a rubbery elastomer homogeneously dispersed in a continuous phase of a hard resin such as a methacrylic resin are the particle size and crosslinking degree of the rubbery elastomer, the graft-polymerizability of the hard resin phase to the rubber phase, and the molecular weight of the hard resin. In fact, the relative superiority and balance of the resin characteristics of the final resin composition are greatly influenced by these factors.
More specifically, where the particle size of the rubbery elastomer is small, the transparency is high but the impact resistance is degraded, and where the crosslinking degree of the elastomer is high, the surface gloss of the final resin composition is good but the impact resistance is poor.
Similarly, the degree of the graft-polymerization of the hard resin phase to the rubbery elastomer has a large influence on the compatibility of the rubbery elastomer with the continuous resin phase and the dispersibility of the rubbery elastomer in the continuous resin phase, and also on the impact resistance, transparency, stress-whitening resistance, surface gloss, and flow processability. When a saturated rubbery elastomer is used, the graft-polymerizability is ordinarily low, and special care should be taken. As the molecular weight of the hard resin is high, the impact resistance is effectively improved but the moldability and surface appearance of the final composition are degraded.
As a means for enhancing the compatibility of the rubbery elastomer with the hard resin component, a method has been proposed in which a methyl methacrylate monomer mixture containing an acrylic acid ester is polymerized in multiple stages in the presence of an alkyl acrylate polymer elastomer (see U.S. Pat. No. 3,562,235). However, this method includes at least four stages and is very complicated.
The present inventors previously proposed a method in which by polymerizing a mixture of an acrylic ester monomer with alkyl acrylate or methacrylate in preparing an acrylic acid ester polymer elastomer by polymerization, the compatibility of the elastomer with the hard resin comprising methyl methacrylate units as the main structural units is increased and an impact-resistant methacrylic resin having an excellent transparency and weatherability is obtained (see Japanese Examined Patent Publication No. 54-18298).
In this method the above-mentioned compatibility is improved and a transparent impact-resistant methacylic resin having an excellent weatherability can be obtained. However, the resin composition obtained according to this method tends to be influenced by the molding condition, in particular the surface gloss of a molded article of this resin composition is readily influenced by the molding conditions.